1. Field of the Invention
The present invention relates to an optical data reproduction apparatus for reproducing data from an optical recording medium and, more particularly, to an optical data reproduction apparatus for performing reproduction scanning of data by moving a reproduction head relative to an optical data recording medium.
2. Description of the Related Art
In an optical recording medium such as an optical card, data is recorded by forming pits (non-reflecting portions) on a track (reflecting portion) at intervals according to recording data. Therefore, a reproduction apparatus detects the position of each pit by moving a reproduction head relative to a recording medium, thereby detecting the interval between each two adjacent pits. In order to accurately detect the pit interval, the relative moving speed between the recording medium and the reproduction head must always be constant.
However, in practice, upon mechanical driving of a recording medium such as an optical disk, an optical card, or the like, a certain speed variation inevitably occurs, and some countermeasure against the speed variation must be taken. In particular, in the case of the optical card, since a large number of parallel tracks extend in the longitudinal direction of the card, the card is linearly and reciprocally moved in its longitudinal direction. Therefore, since acceleration and deceleration are frequently performed, a speed variation occurs more easily than an optical disk which is always in a steady rotation state. Upon reproduction of data near the two ends of each track, the reproduced data are easily influenced by the speed variation.
In order to solve this problem, a conventional optical card reproduction apparatus uses a linear motor which suffers a less speed variation as a driving source for a card, or a linear encoder is attached to control the speed. However, these methods are not preferable since the cost and weight of the apparatus undesirably increase.
A method of detecting the moving speed of a medium from reproduced data, and correcting the reproduced data (pit interval) in accordance with the detected speed is proposed. The moving speed is detected on the basis of a time interval required when a certain pit passes across two detectors which are arranged on a track to be separated by a predetermined distance. With this method, reflection signals from a pit array on a single track are detected by the two detectors. Blank and preamble portions are detected from the output signals from the two detectors, thereby detecting the head position of recording data (a data portion). Then, a synchronization (sync) portion is detected, and the difference between the output timings of the two detectors is calculated by establishing a pit-to-pit correspondence using the last pit in the sync portion as the head pit of the data portion, thereby obtaining speed data. Thereafter, the pit interval (reproduced data) in the output signal from one detector is corrected in accordance with the speed data, thereby obtaining reproduced data which does not include any speed variation.
Note that the sync portion consists of a plurality of bits which are arranged according to a predetermined rule (which does not appear in a modulation rule). For example, the sync portion is recorded by four pits which are arranged at intervals .tau., 4.tau., and 4.tau.. Therefore, in the conventional method, the sync portion is detected by discriminating whether or not the time differences of pits have the above-mentioned relationship .tau., 4.tau., and 4.tau..
However, according to this method, when the moving speed of the card varies, since the pit interval data varies accordingly, a correct sync pattern cannot be detected. When the output signal from the detector suffers omission or addition of pits (detection errors of pits), a pit pattern which is not originally a sync portion may be erroneously recognized as a sync portion. Such erroneous recognition easily occurs when the sync pattern is simple.